finding 6.2 : exposure-risk-from-extreme-precipitation-events

Runoff from more frequent and intense extreme precipitation events will increasingly compromise recreational waters, shellfish harvesting waters, and sources of drinking water through increased introduction of pathogens and prevalence of toxic algal blooms [High Confidence]. As a result, the risk of human exposure to agents of water-related illness will increase [Medium Confidence].

Process for developing key messages: The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors at several workshops, teleconferences, and email exchanges. Authors considered inputs and comments submitted by the public, the National Academies of Sciences, and Federal agencies. For additional information on the overall report process, see Appendices 2 and 3.

Many water-related illnesses are of critical importance globally, such as cholera and hepatitis E virus, and they affect U.S. interests abroad, but the focus of this chapter is to address climate impacts on water-related illnesses of primary importance to human health within the United States. In addition, although climate change has the potential to impact national as well as global seafood supplies, this chapter does not cover these types of impacts because the peer-reviewed literature is not yet robust enough to make connections to human health outcomes in the United States. Even with those constraints, the impacts of climate on water-related illness are regionally or locally specific and may include increased risks as well as benefits. For example, the projected geographic range shifts of some Gambieridiscus species to more northern latitudes may mean that dominant ciguatera fish poisoning toxins enter the marine food web through different species, with increases of toxins in new areas where waters are warming and potential decreases in areas such as the Yucatan and eastern Caribbean Sea.1dfd14e0-eae8-46d9-9c3e-0fa3f0c37da4

New information and remaining uncertainties: Changes in exposure and risk are attributable to many factors in addition to climate. While extreme precipitation and flooding events introduce contaminants and pathogens to water to varying degrees depending on the characteristics of each individual event, they may not always result in increases in exposure due to planning and adaptive actions. There are limited studies on actual projections for changes in illness rates due to increasing frequency or intensity of extreme precipitation events. Uncertainty remains regarding appropriate methods for projecting changes in illness rates, including how to integrate considerations of human behavior into modeling (current methods to assess exposure risk assume similar human behavior across time scales and geography). Methodological challenges are related to 1) baseline case reporting issues (underreporting and underdiagnosis), 2) accounting for the effects of potential adaptation strategies/public health interventions (for example, public service announcements about how to avoid exposure), and 3) accounting for changes in public healthcare infrastructure and access that can reduce the risk of exposure or of illness/death.

Assessment of confidence based on evidence: Based on the evidence, there is high confidence that increasing frequency or intensity of extreme precipitation events will compromise recreational waters and sources of drinking water with pathogens, nutrients, and chemical contaminants from agricultural, wildlife, and urban sources.

There is consistent qualitative evidence that flooding associated with extreme precipitation events and storm surge results in loading of pathogens and nutrients to surface and groundwater (and drinking water distribution systems) through stormwater runoff and sewage overflows. However, other human and social factors modify risk, and there are no national-level studies upon which to draw conclusions regarding quantitative projections of increased exposure. Thus, the limited number of studies supports a medium confidence level that human exposure risk will increase due to changes in extreme events.